Heat-developable photographic light-sensitive materials which undergo heat development process to form a photographic image are disclosed in U.S. Pat. Nos. 3,152,904 and 3,457,075, and D. Morgan and B. Shely, "Thermally Processed Silver Systems", Imaging Processes and Materials, Neblette, 8th ed., Sturge, V. Walworth, A. Shepp, page 2, 1969.
Such a heat-developable photographic light-sensitive material normally comprises a reducible silver source (e.g., organic silver salt), a catalytically active amount of a photocatalyst (e.g., silver halide) and a reducing agent, usually dispersed in a (organic) binder matrix. If necessary, a color toning agent for controlling the tone of silver may be further dispersed therein. The heat-developable photographic light-sensitive material stays stable at ordinary temperatures. The heat-developable photographic light-sensitive material which has been exposed to light is heated to a temperature as high as not lower than 80.degree. C. to allow the reducible silver source (which acts as an oxidizer) and the reducing agent to undergo redox reaction to produce silver. This redox reaction is accelerated by the catalytic action of a latent image produced by exposure. The silver produced by the reaction of the organic silver salt in the exposed area provides a black image which forms an image in contrast to the unexposed area.
However, there have been no techniques for controlling development to provide a higher contrast. It has been desired to develop such a technique.